In vertical drilling, a typical objective is to drill a consistently vertical borehole in a manner which minimizes the number and magnitude of unintended deviations of the borehole from vertical. In directional drilling, a typical objective is to drill a borehole along a predetermined path or paths to reach a subsurface target in a manner which minimizes the number and magnitude of unintended deviations and other unintended directional changes of the borehole.
In either case it is desirable to have the ability to control the drilling direction while drilling, since the existence of unintended directional changes complicates both drilling and completion and can increase the amount of time required to drill and complete the borehole.
Unintended directional changes in a borehole may be due to such causes as the characteristics of the formation being drilled, the characteristics of the drilling string, or to phenomena such as bit walk and reactive torque. The resulting borehole may exhibit crookedness or spiralling, may include doglegs and/or keyseats and may cause increased drag and torque on the drilling string, drilling string failures and production problems. The borehole may also miss an intended subsurface target.
Various options are available for providing steering capability to a drilling tool during drilling in an effort to ensure straightness and/or a desired direction of the borehole.
In directional drilling applications, a first option is to attach a bent-housing or a bent-sub downhole drilling motor to the end of the drilling string as a steering tool. When steering is required (such as, for example, to correct the effects of an unintended directional change) the drilling string can be restrained against rotation and the drilling motor can be pointed in a desired direction and operated for both drilling and steering in a “sliding drilling” mode. When steering is not required, the drilling string and the drilling motor can be rotated together in a “rotary drilling” mode. One advantage to this first option is its relative simplicity. One disadvantage to this first option is that steering is only possible in the sliding drilling mode. A second disadvantage to this first option is that the straightness of the borehole in rotary drilling mode may be compromised by the presence of the bent drilling motor.
A second option for steering in directional drilling applications is the use of a “rotary steerable” drilling system as a steering tool. In a rotary steerable drilling system, the drilling string may be rotated while the drilling tool is being steered either by being pointed or by being pushed in a desired direction either directly or indirectly by a steering device or steering devices. A rotary steerable drilling system may include a component which is non-rotating relative to the drilling string in order to provide a reference point for the desired direction and a mounting location for the steering device or devices. Alternatively, a rotary steerable drilling system may be “fully rotating”.
One advantage to rotary steerable drilling systems is that they can provide relatively high steering accuracy. One disadvantage to rotary steerable drilling systems is that they tend to be relatively expensive and relatively complex apparatus, due in part to the necessity of determining orientations and directions in three dimensions for directional drilling applications.
U.S. Pat. No. 5,168,941 (Krueger et al) describes a drilling system which includes an array of extendable and retractable force transmitting members and pressure members which are actuated in response to positional data from sensors. The force transmitting members and pressure members are hydraulically actuated by electrically operated control valves using drilling fluid as the hydraulic fluid. The actuating pressure is generated by the creation of high pressure and low pressure drilling fluid regimes through the use of throttles either within the tool or in the borehole.
U.S. Pat. No. 5,603,386 (Webster) describes a drilling system which includes an array of extendable and retractable stabilizer blades. The system may be used for vertical well control. When used in vertical well control applications the stabilizer blades are hydraulically actuated in response to movement of ball bearing sensors which form a link in a “hydraulic solenoid” when the tool deviates from vertical. A system of pilot valves is actuated by the hydraulic solenoid in order to extend or retract the stabilizer blades. The actuating pressure is generated using a pump.
For vertical drilling applications, several options for steering tools are disclosed in the prior art for providing steering capability to a drilling tool during vertical drilling of a borehole.
U.S. Pat. No. 2,075,064 (Schumacher et al) describes a steering tool for use in rotary drilling which includes a free swinging pendulum mounted in a barrel, a closure plate positioned at the lower end of the pendulum, and a plurality of discharge ports associated with the closure plate. In operation, deviation of the drilling string from vertical results in blocking of the discharge port adjacent to the low side of the borehole, with the result that drilling fluid flowing through the barrel is preferentially directed against the high side of the borehole to exert a force to direct the drilling string back to a vertical orientation.
U.S. Pat. No. 2,153,680 (Schumacher et al) describes a steering tool for use in rotary drilling in which a plurality of discharge passages are associated with sealing rings located on an exterior surface of a pendulum mounted in a barrel. In operation, deviation of the drilling string from vertical results in the sealing rings sealing the discharge passages adjacent to the low side of the borehole, with the result that drilling fluid passing through the barrel is directed against the high side of the borehole to exert a force to direct the drilling string back to a vertical orientation.
U.S. Pat. No. 3,141,512 (Gaskell et al) describes a steering tool for use in sliding drilling in which a pendulum in a casing is associated with a plurality of potentiometers. In operation, deviation of the drilling string from vertical causes control signals to be generated by the potentiometers, which in turn actuates an electro-hydraulic control valve, which results in energization of one or more pistons located inside the casing and pivoting of a lower casing section relative to an upper casing section to bring the lower casing section in line with the pendulum. The pistons are hydraulically actuated using oil as the hydraulic fluid, which oil is pressurized by a pump.
U.S. Pat. No. 3,243,001 (Vincent) describes a steering tool for use in rotary drilling which includes a pendulum in a housing with a ring at its lower end which functions to selectively expose or block a plurality of ports which are located adjacent to the lower end of the pendulum during the passing of drilling fluid through the housing. The ports communicate with a plurality of conduits and pistons and each conduit is provided with an orifice for providing a pressure drop in the conduit. In operation, deviation of the drilling string from vertical causes the ring to block the port or ports adjacent to the high side of the borehole and expose the port or ports adjacent to the low side of the borehole. Exposure of the port or ports at the low side of the borehole results in actuation by drilling fluid of the piston associated with the port, which in turn causes the piston to exert a force on the inside of he housing to pivot the drilling string relative to the housing in a direction away from the low side of the borehole.
U.S. Pat. No. 3,637,032 (Jeter) and related U.S. Pat. No. Re. 29,526 (Jeter) describe a steering tool for use in rotary drilling which includes a pendulum inclinometer and a compass as direction sensing means which are mounted in a housing and which together rotate relative to the drilling string at the speed of the drilling string and in the opposite direction in order to hold the direction sensing means substantially non-rotative relative to the earth. In operation, deviation of the drilling string either azimuthally or vertically results in actuation of one or more mechanical valves, resulting in selective inflation of bladders by drilling fluid and extension of ribs to impose a lateral force on the drill bit to urge the drilling string back on course.
U.S. Pat. No. 5,314,030 (Peterson et al) describes a steering tool for use in sliding drilling which includes an oscillating pendulum which is mounted on a rotatable drilling shaft. The pendulum is constrained so that it can oscillate only in a single plane. In operation, deviation of the drilling string from vertical results in similar deviation of the oscillating pendulum. The amplitude and phase relationship of the oscillations of the pendulum relative to the angular position of the drilling shaft are sensed with a transducer to produce control signals. The control signals are used to regulate fluid jets from the drill bit, providing preferential flushing to guide the drill bit back to a vertical course.
The AutoTrak™ drilling system, developed by Baker Hughes INTEQ, is an automated steering system for use in sliding drilling to drill vertical wells. The AutoTrak™ system is therefore intended to be used in conjunction with a drilling motor. The AutoTrak™ system includes three extendable and retractable stabilizer pads, inclinometers, microprocessors and internal hydraulic pumps. In operation, deviation of the drilling string from a desired orientation is sensed by the inclinometers and results in activation of the hydraulic pumps. Signals from the inclinometers are provided to the microprocessors which calculate the force required to overcome the deviation. The hydraulic pumps then deliver an extending force to one or more of the stabilizer pads in order to direct the drilling string back to the desired orientation. The VertiTrak™ drilling system, also developed by Baker Hughes INTEQ, is a version of the AutoTrak™ system which has been adapted for use in vertical drilling applications.
The PowerDrive™ drilling system, developed by Schlumberger, is an automated steering system for use in rotary drilling to drill vertical wells. The PowerDrive™ system is a fully rotating rotary steerable system. The PowerDrive™ system includes a bias unit with extendable and retractable pads. The bias unit rotates with the drill string. The extension and retraction of the pads is synchronized with the rotation of the drill string so that the pads are extended and retracted at a consistent rotational orientation. The extension and retraction of the pads is controlled by a control unit which contains self-powered electronics and sensors. The control unit is a “roll-stabilized platform” which maintains a constant orientation by rotating relative to the drill string. The PowerV™ drilling system, also developed by Schlumberger, is a version of the PowerDrive™ system which has been adapted for use in vertical drilling applications.
There remains a need for a steering tool which is relatively easy to construct and maintain and which is relatively simple to operate. There remains a need for a steering tool which does not require electrical sensors or electrically operated valves in order to perform the steering function. There remains a need for a steering tool which can be adapted for use in either rotary drilling or sliding drilling.